MULTIPLEX ADDRESSING OF FERRO-ELECTRIC LIQUID CRYSTAL DISPLAYS

Abstract: The invention provides a ferro-electric liquid crystal display (FLCDs) with reduced voltages requirements for driver circuits. This enables standard drivers circuits designed to rms address twisted nematic type of displays, to be used for FLCDs. Displays are formed by cells containing smectic liquid crystal material. The cell walls are surface treated and carry e.g. row and column electrodes forming an x,y matrix of addressable display elements. The smectic liquid crystal material switches between two states upon application of a dc pulse of appropriate amplitude, polarity, and time. Addressing waveforms are strobe waveforms, e.g. two pulses of opposite polarity in successive time slots, applied to each row in turn. Data waveforms are, e.g. dc pulses of alternate polarity with each pulse lasting one time slot ts. Two data waveforms are needed to switch between the two states; one data waveform is the inverse of the other. Typically a strobe waveform pulse may be 50 volts. In the invention a voltage reduction waveform (VRW) is added to both strobe and data waveforms. This has the effect of reducing the maximum amplitude of voltage needed by the driver circuits, whilst leaving the resultant voltage appearing at a display element at the same value as if VRW were not used. With a reduced voltage requirement, driver circuits previously used for relatively low voltage nematic material type of displays can be used to switch smectic materials.

Description: MULTIPLEX ADDRESSING OF FERRO-ELECTRIC LIQUID CRYSTAL DISPLAYS This invention relates to the multiplex addressing of ferroelectric liquid crystal displays (FLCDs). Such displays may use a chiral smectic C, I, and F liquid crystal material. Liquid crystal display devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Electrode structures on the inner faces of these slides enable an electric field to be applied across the liquid crystal layer thereby changing its molecular alignment. Many different types of displays have been made using nematic and cholesteric liquid crystal material. Both these types of material are operated between an electric field ON state and a field OFF state; i.e. displays are operated by switching an electric field ON and OFF Both nematic and cholesteric material respond to the rms value of applied electric field; they are not polarity sensitive. A more recent type of display uses a ferroelectric chiral smectic C, I, and F liquid crystal material in which liquid crystal molecules adopt one of two possible field ON states depending on the polarity of applied field. These displays are thus switched between the two states by dc pulses of appropriate polarity. In a zero applied field the molecules may adopt an intermediate, configuration depending upon surface alignment treatment. Chiral smectic displays offer very fast switching together with an amount of bistability which depends upon material, liquid crystal material layer thickness, and cell surface alignment processes. Examples of chiral smectic displays are described in G.B. No. 2,163,273; G.B. No. 2,159,635; G.B. NO. 2,166,256; G.B. No. 2.157.451; U.S.A. Patent No. 4,536,059; U.S.A. Patent 4,367,924; G.B. P.A. No 86/08,114 - GB 2.209,610 - P.C.T. No. G.B. 87/00,222; G.B. P.A. No 86/08,115 - GB 2,210,468 - P.C.T. No 87/00,221; G.B. P.A. No. 86/08,116 - GB 2,210,469 - P.C.T. 87/00,220. One known display is formed as an x, y matrix of pixels or display elements produced at the intersections between column electrodes on one wall and row electrodes on the other wall. The display is addressed in a multiplex manner by applying voltages to successive row (x) and column (y) electrodes. There are a number of known systems for multiplex addressing chiral smectic displays; see for example article by Harada et al I98 S.I.D. Paper 8.4 pp 131-134, and Lagerwall et al 198 I.D.R.C. pp 213-221. See also GB 2,173,336-A and GB 2,173.629-A. Multiplex addressing schemes for FLCDs employ a strobe waveform that is applied in sequence down eg a row of electrodes simultaneously with data waveforms applied to eg column electrodes. A characteristic of FLCDs is that they switch on receipt of a pulse of suitable voltage amplitude and length of time of application, ie pulse width, termed a voltage time product V.t. Thus both amplitude and pulse width need to be considered in designing multiplex addressing schemes. To address a large display in a relatively short time requires short pulse widths and a correspondingly high voltage. In a typical display cell the pulse width is 0 to 100 μsec and voltages up to 50 volts need to be switched through drivers circuits to a display. At present the circuitry for driving a large number of electrodes in a display exists for multiplex addressed nematic devices such as the 90° twisted nematic and the 270° super twisted nematic with their relatively low voltage switching requirements, eg peak voltages of +/- 25 volts; see for example H Kawakami, Y Nagae, and E Kaneko, SID Conference Proceedings 1976 pages 50~52. Circuitry capable of handling larger voltage levels are only available with about 64 outputs per circuit chip. Large displays require well over 100 outputs per chip. There is therefore a problem in addressing large FLCD because of the dual requirement to handle large voltage levels and provide a large number of outputs connections. An object of the present invention is to reduce the voltage levels required by multiplex driving circuits to address FLCDs. The above problem is solved according to this invention, by adding an additional waveform to both strobe and data waveforms used to address a FLCD whilst still maintaining a resultant voltage on the material sufficient to cause switching. This allows existing multiplex addressing drive chips designed for low voltage rms addressed displays to be used for multiplex addressing FLCDs. According to this invention a method of multiplex addressing a ferroelectric liquid crystal display formed by the intersections of an m set of electrodes and an n set of electrodes to provide an m x n matrix of addressable display elements comprises the steps of: generating row and column waveforms comprising voltage pulses of differing dc amplitude and sign in sucessive time slots (ts) for applying to the m and n sets of electrodes; multiplex addressing the m and n set of electrodes with the row and column waveforms applied through driver circuits to address each display element; characterised by the step of modifying both row and column waveforms by a voltage reduction waveform, whereby the voltage levels required by the driver circuits are reduced whilst maintaing a sufficient voltage amplitude at display elements to cause switching. According to this invention a multiplex addressed liquid crystal display comprises: a liquid crystal cell including a layer of ferro-electric smectic liquid crystal material contained between two walls each bearing a set of electrodes arranged to form collectively a matrix of addressable display elements; driver circuits for applying data waveforms to one set of electrodes and strobe waveforms to the other set of electrodes in a multiplexed manner, waveform generators for generating data and strobe waveforms of tinipolar pulses in sucessive time slots (ts) for applying to the driver circuits, means for controlling the order of data waveforms so that a desired display patern is obtained, characterised by:- means for modifying the data and strobe waveforms so that voltage levels applied to the driver circuits are reduced whilst maintaining the resultant voltage level appearing at the addressable intersections. The unipolar pulses are substantially dc pulses of required amplitude and polarity, each lasting for one time slot (ts). Brief description of drawings: One form of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figures 1, 2, are plan and section views of a liquid crystal display device; Figure 3 is a stylised perspective view of a layer of aligned liquid crystal material showing a chevron type of molecular layer alignment; Figure 4 is a stylised sectional view of part of Figure 3 to a larger scale, one of several possible director profiles possible with the chevron structure; Figure is a graph of applied voltage pulse width against voltage amplitude, showing switching characteristics for different amounts of applied ac bias for a material showing a voltage time (v.t) minimum; Figure 6 is a block diagram of part of Figure 1 showing inputs to and outputs from display driver circuits; Figures 7 and 13 are prior art waveform diagrams showing strobe and data pulses used in addressing an x, y matrix display; Figures 8 to 12, and 14 to 20 are waveform diagrams showing the invention applied to different addressing systems; Description of preferred embodiments. The cell 1 shown in Figures 1, 2 comprises two glass walls, 2, 3. spaced about 1-6 μm apart by a spacer ring 4 and/or distributed spacers. Electrode structures 5. 6 of transparent tin oxide are formed on the inner face of both walls. These electrodes may be of conventional row ■ (x) and column (y) shape, seven segment, or an r-0 display. A layer 7 of liquid crystal material is contained between the walls 2, 3 and spacer ring 4. Polarisers 8, 9 are arranged in front of and behind the cell 1. The alignment of the optical axis of the polarisers 8, 9 are arranged to maximise contrast of the display; ie approximately crossed polarisers with one optical axis along one switched molecular direction. A d.c. voltage source 10 supplies power through control logic 11 to driver circuits 12, 13 connected to the electrode structures 5, 6, by lead wires 14, 1 . The device may operate in a transmissive or reflective mode. In the former light passing through the device e.g. from a tungsten bulb 16 is selectively transmitted or blocked to form the desired display. In the reflective mode a mirror 17 is placed behind the second polariser 9 to reflect ambient light back through the cell 1 and two polarisers. By making the mirror 17 partly reflecting the device may be operated both in a transmissive and reflective mode with one or two polarisers. Prior to assembly the walls 2, 3 are surface treated by spinning on a thin layer of a polymer such as a polyamide or polyimide, drying and where appropriate curing; then buffing with a soft cloth (e.g. rayon) in a single direction RI, R2. This known treatment provides a surface ali nmpnt for liquid rrystal tnolpf-tiles. The molecules (as measured in the nematic phase) align themselves along the rubbing direction RI, R2, and at an angle of about 0° to 15° to the surface depending upon the polymer used and its subsequent treatment; see article by S Kuniyasu et al, Japanese J of Applied Physics vol 27, No 5. May 1988, ppδ27-829. Alternatively surface alignment may be provided by the known process of obliquely evaporating eg. silicon monoxide onto the cell walls. The surface alignment treatment provides an anchoring force to adjacent liquid crystal materials molecules. Between the cell walls the molecules are constrained by elastic forces characteristic of the material used. The material forms itself into molecular layers 20 each parallel to one another as shown in Figures 3, 4, which are specific examples of many possible structures. The Sc is a tilted phase in which the director lies at an angle to the layer normal, hence each molecular director 21 can be envisaged as tending to lie along the surface of a cone, with the position on the cone varying across the layer thickness, hence the chevron appearance of each macro layer 20. Considering the material adjacent the layer centre, the molecular director 21 lies approximately in the plane of the layer. Application of a dc voltage pulse of appropriate sign will move the director along the cone surface to the opposite side of the cone. The two positions Dl, D2 on this cone surface represent two stable states of the liquid crystal director, ie the material will stay in either of these positions Dl, D2 on removal of applied electric voltage. In practical displays the director may move from these idealised positions. It is common practice to apply an ac bias to the material at all times when information is to be displayed. This ac bias has the effect of moving the director and can improve display appearance. The effect of ac bias is described for example in Proc 4th IDRC 1984 pp 217-220. Display addressing scheme using ac bias are described eg in GB patent application number 90.17316.2, PCT/GB 91/01263, J R Hughes and E P Raynes. The ac bias may be data waveforms applied to the column electrodes 1 • Suitable materials include catalogue references BDH-SCE 8, ZLI-5014-000, available from Merck Darmstadt, and those listed in PCT/GB88/0100 , WO 89/05025, and:- 19.6 CM8 (49 CCl + 51% CC4) + 80.4% H. Me CCl = C 8 H 17 -(θ)-(θ)-OOC*H C 6 H 13 Me Me I I CC4 = CH 2 C*HC 2 H 5 -<^θ - θ -COO- θ)-CH 2 C*H C 2 H 5 H, M. M. M, ( 1 1 ) F M 2 = C 8 H 17 0 -{0>-(0>-C00-<0>-C 5 H 11 F Another mixture is LPM 68 = HI (49-5%), AS 100 (49-5%). IGS 97(1%) HI = MB 8.5F + MB 80. F + MB 70.7F (1 : 1 : 1) ASlOO = PYR 7.09 + PVR 9-09 (1 : 2) F MB 8.5F = CH 3 (CH 2 ) ? -{θ)-(θ)~ C0 2 -(θ)- (CH 2 )„ CH 3 MB 80. F = CH 3 (CH 2 >7 0 " D-D " C0 Λ°) ~ {CH 2 4 CH 3 MB 70.7F = CH 3 (CH 2 ) 6 - 0 " θ)-(θ)- C0 2 -(θ)- (CH 2 ) 6 CH 3 PYR 7-09 = CH 3 (CH 2 ) 6 ^N PYR 9-09 = CH 3 (CH 2 ) 8 ^^ -<0 0 -(CH 2 ) 8 CH 3 IGS 7 = CH j. ( * CH- <- * ).7, CO- * - C j*H - CHfCH j- .. CN The switching characteristic of pulse width against applied voltage for one material LPM68 in a layer 1.7um thick at 20 β C is shown in Figure - For values of voltage time products (v.t) in the area below the curves the liquid crystal material will not switch. For v.t products above the lines the material will switch. As shown the curve varies somewhat with the level of a.c bias applied; this is described later. Also- the curves vary with the relative amplitude values of two strobe pulses as described in WO 89/05025. Thus in determining switching characteristics for a given material, the product v.t, the shape of resultant waveforms of a pixel, the amount of a.c bias, and the material temperature must be taken into account. Some liquid crystal materials have differently shaped v.t characteristics. For example some materials do not show the minimum seen in Figure 5. but merely a decreasing curve of pulse width with increasing voltage. For maximum contrast in most two polariser devices, it is desirable for the apparent cone angle, or the angle between the director in the two switched states to be about 45". One of the polarisers is aligned parallel to one of the two switched director positions; the second polariser is aligned perpendicular to the first polariser. Alternatively, as described in GB 9127316, and PCT/GB 9202368 the polarisers may be rotated from the crossed position to improve contrast between the two switched states. Figure 7 shows waveforms used in a prior art addressing scheme to switch a four row by four column array. As shown open circles may be defined as OFF pixels and solid circles as ON pixels. A strobe waveform is applied to each of rows RI to R4 in turn and comprises a zero for one time slot ts followed by a dc pulse of -Vs for one time slot; rows not receiving the strobe pulse receive a zero voltage. Thus for row RI the applied waveform is zero volts in tsl, -Vs in ts2, followed by zero volts for the time slots ts3 to tsδ. The time tsl to tsδ is termed a field time" and ■ is "equal to N x 2-ts," * where N- is- the number of lines in a display. For row R2 the applied waveform is zero in tsl, ts2, then the strobe waveform of zero volts in ts3 and -Vs in ts4, and zero volts for the remainder of the frame, ie ts5 to tsδ. Similarly for rows R3 and R4 the strobe waveform is applied during ts5, ts6 and ts7, tsδ respectively with zero volts at the other time slots. The opposite is then applied for a further field, namely a zero for one ts, a +Vs for one ts, and zero for the remainder of the field time. Two fields are necessary to completely switch the array and this time is termed the frame time; displays are continually addressed by successive frame. The first field (or odd number of field) switches all required pixels to the ON state and the second field (or even number of field) switches all required pixels to the OFF state. The waveforms applied to the columns are termed data ON and data OFF waveforms; each comprises alternate pulses of +/- Vd with a pulse length of ts. The data ON and data OFF are of opposite sign. The resultant of strobe pulses and data pulses at pixels marked as A, B, C, D are shown and are termed resultant waveforms. The resultant waveforms are the voltage levels across the liquid crystal material. Pulses marked with a single hatching, of amplitude Vs+Vd and length ts, do not switch the material. Pulses marked with (double) cross hatching, of amplitude Vs-Vd, switch the material when operating in the v.t minimum mode (Figure 5) • As shown pixels A and D switch in the first field whilst those marked B, C switch in the second field. In the scheme shown by Figure 7 the value of Vs is 5Vd. Typically Vs = 0 volts. Addressing schemes of the present invention use strobe and data waveforms with approximately equal maximum voltage levels yet apply similar peak resultant voltages to the liquid crystal material. The effect of this is to reduce the voltage requirements on the driver circuits allowing components presently-used- in multiplex rms addressing field effect liquid crystal displays to be used in addressing FLCDs. In the following Figures 7 to 12 and 14 to 20 various prior art addressing schemes are modified by additional waveforms applied to both strobe and data waveforms to provide lower voltage levels at the drivers. Figure 8 shows a strobe waveform having balanced strobe pulses of first a +Vs for one time slot ts immediately followed by -Vs for one ts for the first field. Polarity is reversed, and in the second field the strobe is -Vs followed by +Vs. Line address time is 2ts. A voltage reduction waveform, VRW, comprises pulses of +(Vs-Vd)/2 for ts followed by -(Vs-Vd)/2 for ts alternately for one field. Polarity is reversed for the second field. The resultant waveform for each row Rw is the difference between strobe waveform and the VRW. This gives the waveform shown which has four voltages levels +(Vs+Vd)/2, +(Vs-Vd)/2, -(Vs-Vd)/2, and -(Vs+Vd)/2. The basic data waveforms ON and OFF are alternate pulses of +/- Vd in each time slot ts. Again a VRW is alternate pulses of +/- (Vs-Vd)/2. The resultant data waveforms Rd aDDlied to each column, are waveforms with four voltage levels of +(Vs+Vd)/2, +(Vs-3Vd)/2, -(Vs-3Vd)/2, and -(Vs+Vd)/2. The resultant waveform at a pixel is the combination of Rw and Rd which has exactly the same waveform, both shape and amplitude, as if the strobe and data waveforms alone had been applied. The result is correct switching as required, but with the maximum voltage applied by a driver reduced from Vs to (Vs+Vd)/2; in a typical case this may be a reduction from 50 to 30 volts, where Vs = 50 volts and Vd = 10 volts. Figure 9 shows waveforms for addressing the first line in a modified monopulse address scheme. The strobe waveform is first a zero voltage in the first ts followed by a single pulse of -Vs in the second time slot, and then zero pulses in the time slots remaining in the first field. In the second field the strobe pulse is +Vs. A row voltage reduction waveform is a single level of -(Vs-Vd)/2 for N x 2ts for the first field and (Vs-Vd)/2 for the second field. The resultant row waveform has four voltage levels, (Vs-Vd)/2, -(Vs+Vd)/2, -(Vs-Vd)/2, and +(Vs+Vd)/2. Data waveforms are as in Figure 7, alternate pulse of +/- Vd. The data VRW is a -(Vs-Vd)/2 in the first field and +(Vs-Vd)/2 in the second field. The resultant data ON and OFF waveforms have four voltage levels +(Vs+Vd)/2, +(Vs-3Vd)/2, -(Vs-3Vd)/2, and -(Vs+Vd)/2. Resultant waveforms at a pixel are the same values as would be obtained without the strobe and data VRW being employed. Figure 10 shows waveforms for addressing the first and fourth line in an addressing scheme modified from that described in GB9017316. The basic strobe waveform is a zero for the first ts then +Vs for the second ts. In this particular scheme the +Vs pulse is extended for a further ts whilst the start of the strobe waveform is applied to the second row. The reason the strobe waveform starts with a zero pulse is that each pixel is addressed by the resultant of the first (zero) and second (non-zero) strobes pulses in combination with the first and second data pulses. As explained in GB 9017316, whether or not a larger pulse switches depends upon the amplitude and sign of the preceeding smaller pulse. A strobe VRW is -(Vs-Vd)/2 for the first ts followed by +(Vs-Vd)/2 for the remainder of the first field. In the second field the polarity is inverted. The resultant strobe waveform is shown for rows 1 and 4; it has the same four voltages levels as Figure 9. Basic data ON and OFF are alternate pulses of Vd opposite polarity; data ON is the inverse of data OFF. The data VRW is the same as the strobe VRW. The resultant data ON and OFF waveforms are as shown with four voltage levels as in Figure 9. Resultant waveforms at a pixel are the same values as would be obtained without the strobe and data VRW being employed. Figure 11 is similar to Figure 10 except that the strobe pulse of Vs is further extended into the address time of the next row. The strobe and data VRW are as in Figure 10. Strobe, data, and pixel resultant waveforms are as shown. Again waveforms at a pixel are the same values as would be obtained without the strobe and data VRW being employed. Due to the length of the strobe pulse, the VRW can not accommodate this and so it is necessary to have a dummy line, ie the display will be N lines but only N-l may be used. In the above examples, Figures 8 to 11, the VRW was of amplitude (Vs-Vd)/2. As an alternative the amplitude could have been Vs/2, resulting in a higher peak to peak column voltage of Vs+2Vd. Two such examples of this are shown in Figure 12 which has basic strobe and data waveforms identical to those of Figure 9« The first example in Figure 12 has a VRW identical to that in Figure 9 but with an amplitude of + and - Vs/2. The resultant strobe waveform has two voltage levels of +Vs/2 and -Vs/2. The resultant data waveform has four voltage levels of +(Vs/2)+Vd, (Vs/2)-Vd, -( (Vs/2)-Vd) , and -((Vs/2)+Vd). The second example in Figure 12 has data waveforms having pulses of +/-Vs/2 with each pulse lasting one ts. The shape of the resultant strobe and data waveforms are different from the first example in Figure 12, but the number and values of the voltage levels is unchanged. The above examples shown in Figures 8-12 employ strobe pulses of opposite polarity and address a complete display in two field making one frame. A known alternative addressing scheme employs a strobe blanking pulse followed by a switching pulse. The blanking pulse is arranged to be of sufficient amplitude and width that it always switches a pixel. The following strobe pulse selectively switches those pixels needing to be in a different state than that switched by the blanking pulse. An advantage of blanking pulse schemes is that the whole display is addressed by a single scan of the strobe waveforms, thereby halving the display address time. The blanking can be done on a line by line basis, the most common, a block of lines at a time, or the whole display (a whole page) at a time. Figure 13 shows a prior art addressing scheme employing blanking pulses; it does not have any voltage reduction waveforms. The strobe waveform for row RI comprises a blanking pulse of amplitude -Vb for a time of 2ts. The selective switching strobe is first a zero voltage for one ts followed by +Vs for one ts. The line blanking time and line addressing time is 2ts. Also shown is the strobe waveform applied to row R2. Data ON and data OFF waveforms are alternate pulses of +/- Vd each lasting ts. Resultant waveforms at pixels required to be ON and OFF are shown for row 1 column 1 (R1C1) and R2C2. In R1C1 the blanking pulse has switched pixels but the strobe in ts4 has not reversed the state. In R2C2 the pixels have been switched by the blanking pulse, then switched to the opposite state by the strobe pulses. The blanking pulse and strobe pulse do not usually balance; therefore the row waveform polarity is periodically reversed to maintain d.c balance. Figure 14 shows a blanked monostrobe addressing scheme with VRW. Additionally alternate rows have polarity reversal in the strobe waveform. Furthermore the strobe waveforms are polarity inverted, eg in alternate frames to give a net zero dc. To preserve the single polarity excursions of the row waveform when the blanking pulse extends into the previous field it is necessary to have an even number of rows. To preserve the single polarity excursions of the row waveform it is necessary for the blanking pulse to precede the strobe pulse by an odd number of rows. Strobe waveforms for rows RI. R2. R are shown; they are similar to those of Figure 13 but with a polarity reversal in R2. The RI blanking pulse is -Vb for 2ts, followed by a zero for one ts then +Vs for one ts. Data ON and data OFF waveforms are as in Figure 13 and comprise alternate pulses of +/-Vd each lasting ts. A VRW comprises -(Vb-Vd)/2 for 2ts and (Vs-Vd)/2 alternately. The resultant row waveforms Rs and resultant column waveforms Rd are shown for RI, R2, R3, Cl, and C2. Each resultant strobe and, data waveform has four amplitude levels of (Vs+Vd)/2, (Vs-Vd)/2, -(Vb-Vd)/2, -(Vb+Vd)/2. Resultant waveforms at pixels R1C1, R2C2, R1C2 are shown; their shape is the same as those in Figure 13- Therefore the display switches in the same manner as that of Figure 13 but with lower peak voltages in the row drivers. Figure 15 shows an addressing scheme where a whole page is blanked to OFF at the same time, then selected pixels switched to ON. The strobe waveform is shown for RI, R2. All strobe waveforms have a blanking pulse of Vb/2 applied in time slots tsl and ts2 which switches all pixels to one state. A strobe pulse of zero for one ts and then -Vs for one ts is then applied to each row in turn. Data ON and data OFF waveforms are -Vb/2 in time slots tsl and ts2, then alternate pulses of +/- Vd of width one ts. A VRW has zero voltage for time slots tsl, ts2, then a constant -(Vs-Vd)/2 for the remainder of the field. Resultant strobe and data waveforms are shown for RI, R2, Cl, C2. Resultant voltages at pixels R1C1 and R2C2 are shown; again the voltages are the same as if the VRW had not been applied to strobe and data waveforms. Both pixels switch during tsl, ts2 whilst the blanking level of +Vb is applied. Pixel R1C1 switches during ts4 during application of -(Vs-Vd) because it is immediately preceeded by -Vd. In contrast r-ixel R2C2 does not switch during ts6 whilst receiving -(Vs+Vd) because it is immediately preceeded by +Vd. The scheme shown in Figure 15 is unsuitable for displays which are frequently updated because of the recurring blank screen. The concept can be extended to counter this problem by blanking a block of lines at a time. These would be selected by applying a +Vb/2, during a blanking period, to those rows to be blanked and -Vb/2 to all other rows, all columns receive -Vb/2. The concept can be thus further extended,to blank line by line by introducing a blanking period between every line address period. Figure 16 shows a line blanking scheme. In this the basic strobe waveform is a conventional monostrobe waveform at alternate line address periods, at ts3, ts4, ts7. ts8, .... etc. Between times the basic strobe waveform is a blanking waveform of +/- Vb/2 for 2.ts in time slots tsl, ts2, ts5, ts6, ... etc; Vb = Vs. Similarly the basic data ON and data OFF waveforms are twin pulses of +/- Vd in time slot ts3, ts4, ts7, ts8, ... etc. Between time the data waveforms are blanking pulses of Vb/2 during time slots tsl, ts2, ts5, ts6, ... etc. Basic strobe data waveforms are shown for RI, R2, Cl, C2. A VRW has a voltage of -Vs/2 for pairs of time slots ts3, ts4, ts7. tsδ ...etc. The resultant strobe waveform has two voltage levels, +/- Vs/2. The resultant data waveforms have three voltages levels, (Vs/2)+Vd, (Vs/2)-Vd, -Vs/2. Resultant waveforms at pixels R1C1 and R2C2 are shown. The scheme of Figure 16 provides a reduction in row peak voltage from 3Vs/2 to 2Vs, and column voltage peak of Vs+Vd. This is of benefit providing 3Vs/2 > Vs+Vd, ie Vs > 2Vd. As an alternative, the VRW aπinl -! hiH nav he - IVa-Vή ) / ~ . The line by line blanking of Figure 16 results in an overall doubling of the frame time when all the blanking periods are taken into account. There is therefore no speed improvement over the two-field case of Figure 12. It does have application though for the cases previously referred to where a single polarity blanking pulse is required. This could be achieved in the example of Figure 16 by making the blanking pulse amplitude Vs/2 thus removing the need to periodically invert all row polarities to maintain d.c. balance. Such a scheme with blanking voltages of Vs/2 is shown in Figure 17. Apart from the amplitude of blanking pulse the scheme of Figure 17 is the same as in Figure 16. The peak voltage for the rows is Vs, whilst that for the columns is 3Vs/2 + Vd/2. A disadvantage of the schemes of Figure 16, 17 is that there are many periods of zero volts in the resultant waveform at each pixel. This reduces the amount rms of a.c. voltages and hence the amount of a.c. stabilisation on the device. The technique of a.c. stabilisation is a known technique which improves the contrast observed between the ON and OFF states. Both amplitude and frequency contribute to a.c. stabilisation. Improved a.c stabilisation can be provided by introducing an a.c. component into the blanking waveform as shown in Figures lδ, 19. In Figure Iδ the data ON and data OFF have a pulse of -( (Vs/4)+Vd) for one ts in slot periods tsl, ts5, ts9. ••• etc and a zero pulse in time slots ts2, ts6, tslO, ... etc. Otherwise the data ON and OFF and the strobe waveform are as in Figure 17. In the resultant pixel waveforms of Figure lδ there are no time slot where a zero voltage appears. Thus a.c. stabilisation, and hence display contrast, is improved. Figure 19 differs from Figure lδ in the shape of data ON and OFF waveforms. In Figure 19 there are -( (Vs/4)+Vd) pulses in the first half of tsl, ts2, ts5, ts6, ts9, tslO, ... etc. Otherwise the strobe and data waveforms are as shown in Figure 17. The resultant pixel waveforms are different from those of Figure 17 and have a higher frequency a.c componen . Figure 6 shows a schematic view of row and column drivers 12, 13 supplied with different voltages from a resistive chain 25. This chain has a voltage supply Vee, a variable resistor 26 and a series of resistors all in series. Voltage outputs are VL1 to VL6. The row driver shown is a Texas (RTM) TMS 3 91 having inputs: cp, supplied by a clock at a maximum of lOOkHZ; Data in, a serial input of * 0* and 'I'; and a control input M. Additionally there are voltage level inputs of VL1, VL6, VL5, and VL2. There are δO parallel outputs which connect to rows RI to RδO of the cell 1. Inside the driver 12 is a serial-in, parallel-out shift register 27 which receives its input from Si and is clocked by SC. Each stage of the shift register 27 connects to one of the outputs 14. The voltage appearing on a given output depends upon the value, a logic '0' or '1' , plus the value of the signal M, a logic '0' or '1', as set out in the Truth Table 1 below. The column driver 13 shown is a Texas (RTM) TMS 3492 having inputs:- SC clocking at 6.5 MHz maximum; control M of logic '0 * and '1'; Data in of 4-bit numbers; and four voltage levels VL1, VL3, VL4, VL2. There are 80 outputs 15 to the column electrodes of the cell 1. Within the driver 13 is a serial-in parallel-out 80 stage shift register 2δ whose parallel outputs are fed into an 80 cell latch 29. Each cell o.f the latch 29 connects with one of the 80 outputs 15• The voltage appearing on a given output depends upon the value of the logic 0 or 1 in a latch cell, plus the logic value of M, as set out in the Truth Table 1 below. Truth Table 1 Practical considerations of these driver chips give a maximum voltage of 40 volts. An example of available voltage levels is:- Operation of the drivers 12, 13, to display the pattern seen on Figure 7 namely a 4 x 4 array, by the scheme shown in Figure 8 will now be described. The pattern to be displayed is that the following pixels should be in a DOWN state:- R1C2, R1C3, R2C3, R3C2, R3C4, R4C3, R4C4; all other pixels are in an UP state. The terms DOWN and UP are arbitrary, but correspond with switching by negative and positive pulses respectively. Figure 20 shows basic row and column waveforms, and those basic waveforms as modified by the VRW seen in Figure 8. As seen more clearly in the basic row waveforms a pair of strobe pulses, of time length 2ts = a line address period, is applied to each row RI to R4 in turn. During each line address period the required data UP or data DOWN waveform must be applied to each column Cl to C4 to switch pixels in that line being addressed. Assume both row and column drivers 12, 13 have been preloaded with logic 0 throughout. The number 0110 is loaded into shift register of the column driver 13; note this matches the pattern in row RI. The logic 1 is loaded into shift register of the row driver 12, and simultaneously the contents of the column shift register transferred into the latch 29- The logic states of the shift register stages corresponding to row outputs and latch stages corresponding to column outputs are:- These logic values remain for the first line address period, ie time slots tsl, ts2. To produce two different voltage levels, one in tsl and another in ts2, the value of control input M is changed between logic 0 and 1. During this time the column shift register is loaded with data fo-r the next (R2) line address, ie the number 0010. The logic 1 in the row shift register is clocked along one stage, and data in the column register transferred to the latch. The logic states of the shift register stages corresponding to row outputs and latch stages corresponding to column outputs are:- Modulate M from 0 to 1 to produce two voltage levels at each driver output; this occurs in time slots ts3. ts4. During this time ts3, ts4, the column shift register is loaded with data for the next line address, ie the number 0101. The logic 1 in the row shift register is clocked along one stage, and data in the column register transferred to the latch. The logic states of the shift register stages corresponding to row outputs and latch stages corresponding to column outputs are:- Modulate M from 0 to 1 to produce two voltage levels at each driver output; this occurs in time slots ts5, ts6. During this time ts5. ts6, the column shift register is loaded with data for the next line address, ie the number 0011. The logic 1 in the row shift register is clocked along one stage, and data in the column register transferred to the latch. The logic states of the shift register stages corresponding to row outputs and latch stages corresponding to column outputs are:- Modulate M from 0 to 1 to produce two voltage levels at each driver output; this occurs in time slots ts7, ts8. This sequence is repeated for the second field, but with the values of M inverted. Tables 2 to below show the values of input data D in each row driver shift register stage and column driver latch stage; the value of M; and the value of the row and column driver output (the VL number) during each time slot ts, both in the first and second fields. The first number in the table indicates the row being addressed. Table 3 Second field, lo ic values on rows Rl to R4. Table 4 First field, logic values on columns Cl to C4. Table 5 Second field, logic values on columns Cl to C4 The addressing schemes shown in Figures 7-20 may also be implemented in a similar manner to that shown the above tables 2-5-

Claims: Claims 1. A method of multiplex addressing a ferroelectric liquid crystal display formed by the intersections of an m set of electrodes and an n set of electrodes to provide an m x n matrix of addressable display elements comprising the steps of: generating row and column waveforms comprising voltage pulses of selected dc amplitude and sign in sucessive time slots (ts) for applying to the m and n sets of electrodes; multiplex addressing the m and n set of electrodes with the row and column waveforms applied through driver circuits to address each display element; characterised by the step of modifying both row and column waveforms by a voltage reduction waveform, whereby the voltage levels required by the driver circuits are reduced whilst maintaining a sufficient voltage amplitude at display elements to cause switching. 2. A multiplex addressed liquid crystal display comprising: a liquid crystal cell including a layer of ferro-electric smectic liquid crystal material contained between two walls each bearing a set of electrodes arranged to form collectively a matrix of addressable display elements; driver circuits for applying data waveforms to one set of electrodes and strobe waveforms to the other set of electrodes in a multiplexed manner, waveform generators for generating data and strobe waveforms of unipolar pulses in sucessive time slots (ts) for applying to the driver circuits, means for controlling the order of data waveforms so that a desired display pattern is obtained, characterised by:- means for modifying the data and strobe waveforms so that voltage levels applied to the driver circuits are reduced whilst maintaining the resultant voltage level appearing at the addressable intersections.

Inventors: GRAHAM ALISTAIR, HUGUES JONATHAN RENNIE, TOWLER MICHAEL JOHN, GRAHAM, ALISTAIR, HUGUES, JONATHAN, RENNIE, TOWLER, MICHAEL, JOHN
Applicants: GRAHAM ALISTAIR, HUGUES JONATHAN RENNIE, SECR DEFENCE, TOWLER MICHAEL JOHN, GRAHAM, ALISTAIR, HUGUES, JONATHAN, RENNIE, THE SECRETARY OF STATE FOR DEFENCE, TOWLER, MICHAEL, JOHN
Application year: 1994
Main classification: G09G3
IPC codes: G02F1, G09G3